The present invention relates generally to pressure controlled well tools, and more specifically relates to methods and apparatus for selectively "locking out" or preventing operation of selected pressure controlled well tools until such time as operation is desired.
Many types of well tools are known which are responsive to pressure, either in the annulus or in the tool string, in order to operate. For example, different types of tools for performing drill stem testing operations are responsive to either tubing or annulus pressure, or to a differential therebetween. Additionally, other tools such as safety valves or drill string drain valves may be responsive to such a pressure differential.
Such well tools typically have some member, such as a piston, which moves in response to the selected pressure stimuli. Additionally, these well tools also typically have some mechanism to prevent movement of this member until a certain pressure threshold has been reached. For example, a piston may be either mechanically restrained by a mechanism such as shear pins or similar devices; whereby the pressure must exceed the shear value of the restraining shear pins for the member to move. Alternatively, a rupture disk designed to preclude fluid flow until a certain threshold pressure differential is reached may be placed in a passage between the movable member and the selected pressure source. Each of these techniques is well known to the art.
Disadvantages may be found where multiple pressure operated tools are utilized in a single tool string. Conventional methods and apparatus for operating two tools in a tool string from the same pressure source (i.e., for example, the well annulus) are to establish the tool string such that the operating pressures for the tool to be operated second are at a pressures value greater than that required to operate the first tool. In some circumstances, this can present a disadvantage in that the releasing and operating pressure for the second-operated tool may be required to be higher than would be desirable. For example, in the above-stated example, it could be undesirable to apply the degree of pressure to the well annulus which might be necessary to operate the second-operated tool.
Additionally, in some types of tools it would be desirable to have a well tool operate in response to a specific and predetermined pressure differential for use when conditions in the well have changed. For example, where a tool is to be operated in response to pressure.
Accordingly, the present invention provides a new method and apparatus whereby a pressure operated well tool may be restricted from operation, and may be selectively enabled for operation while minimizing or eliminating pressure applications required to achieve such enabling; and whereby pressure previously applied to a pressure source may be stored in a well tool and used to facilitate operation of the well tool at a desired pressure differential.